Urinary Bladder

THE BLADDER

 A tube, or ureter, leads from each kidney to the urinary bladder. The bladder empties through the urethra, a tube leading to an external open- ing called the meatus. The bladder, which functions as a collecting and temporary storage point for urine, expands to accommodate increasing amounts. With the accumulation of about half a pint, reflex contractions lead to a desire to urinate, or micturate. 

The contractions stimulate pressure receptors in the muscles of the bladder wall, from which nervous impulses go to the brain. When it is convenient to urinate, the brain sends out signals which cause the bladder's external sphincter to relax.

The signals also set up a whole series of other events, including holding of the breath, forcing of the diaphragm down, and contraction of the abdominal wall, which in- crease pressure on the bladder and help it void its accumulated urine. Such is the complexity of the process that it is hardly any wonder that most children are walking before they become able to urinate with con- trolled competence. 

Normal daily production of urine may range from two to three pints. In some diseases, such as diabetes insipidus, the quantity is increased; in others, fever and diarrhea decrease it. Tea, coffee, alcohol, excitement, and nervousness increase urine output; hot water decreases it. Whatever the actual liquid output, a day's urine generally contains about two ounces of solids.